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Update tcp.txt to fix mandatory congestion control ops and default CCA selection. Also, fix comment in tcp.h for undo_cwnd. Signed-off-by: Anmol Sarma <me@anmolsarma.in> Signed-off-by: David S. Miller <davem@davemloft.net>
102 lines
3.9 KiB
Plaintext
102 lines
3.9 KiB
Plaintext
TCP protocol
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============
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Last updated: 3 June 2017
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Contents
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========
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- Congestion control
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- How the new TCP output machine [nyi] works
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Congestion control
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==================
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The following variables are used in the tcp_sock for congestion control:
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snd_cwnd The size of the congestion window
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snd_ssthresh Slow start threshold. We are in slow start if
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snd_cwnd is less than this.
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snd_cwnd_cnt A counter used to slow down the rate of increase
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once we exceed slow start threshold.
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snd_cwnd_clamp This is the maximum size that snd_cwnd can grow to.
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snd_cwnd_stamp Timestamp for when congestion window last validated.
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snd_cwnd_used Used as a highwater mark for how much of the
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congestion window is in use. It is used to adjust
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snd_cwnd down when the link is limited by the
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application rather than the network.
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As of 2.6.13, Linux supports pluggable congestion control algorithms.
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A congestion control mechanism can be registered through functions in
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tcp_cong.c. The functions used by the congestion control mechanism are
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registered via passing a tcp_congestion_ops struct to
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tcp_register_congestion_control. As a minimum, the congestion control
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mechanism must provide a valid name and must implement either ssthresh,
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cong_avoid and undo_cwnd hooks or the "omnipotent" cong_control hook.
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Private data for a congestion control mechanism is stored in tp->ca_priv.
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tcp_ca(tp) returns a pointer to this space. This is preallocated space - it
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is important to check the size of your private data will fit this space, or
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alternatively, space could be allocated elsewhere and a pointer to it could
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be stored here.
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There are three kinds of congestion control algorithms currently: The
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simplest ones are derived from TCP reno (highspeed, scalable) and just
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provide an alternative congestion window calculation. More complex
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ones like BIC try to look at other events to provide better
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heuristics. There are also round trip time based algorithms like
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Vegas and Westwood+.
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Good TCP congestion control is a complex problem because the algorithm
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needs to maintain fairness and performance. Please review current
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research and RFC's before developing new modules.
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The default congestion control mechanism is chosen based on the
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DEFAULT_TCP_CONG Kconfig parameter. If you really want a particular default
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value then you can set it using sysctl net.ipv4.tcp_congestion_control. The
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module will be autoloaded if needed and you will get the expected protocol. If
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you ask for an unknown congestion method, then the sysctl attempt will fail.
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If you remove a TCP congestion control module, then you will get the next
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available one. Since reno cannot be built as a module, and cannot be
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removed, it will always be available.
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How the new TCP output machine [nyi] works.
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===========================================
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Data is kept on a single queue. The skb->users flag tells us if the frame is
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one that has been queued already. To add a frame we throw it on the end. Ack
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walks down the list from the start.
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We keep a set of control flags
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sk->tcp_pend_event
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TCP_PEND_ACK Ack needed
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TCP_ACK_NOW Needed now
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TCP_WINDOW Window update check
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TCP_WINZERO Zero probing
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sk->transmit_queue The transmission frame begin
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sk->transmit_new First new frame pointer
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sk->transmit_end Where to add frames
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sk->tcp_last_tx_ack Last ack seen
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sk->tcp_dup_ack Dup ack count for fast retransmit
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Frames are queued for output by tcp_write. We do our best to send the frames
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off immediately if possible, but otherwise queue and compute the body
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checksum in the copy.
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When a write is done we try to clear any pending events and piggy back them.
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If the window is full we queue full sized frames. On the first timeout in
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zero window we split this.
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On a timer we walk the retransmit list to send any retransmits, update the
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backoff timers etc. A change of route table stamp causes a change of header
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and recompute. We add any new tcp level headers and refinish the checksum
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before sending.
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